Russia is said to have secretly deployed cruise missile in violation of a landmark arms control treaty that helped seal the end of the Cold War, US administration officials say. The move presents a major challenge for President Trump, who has vowed to improve relations with President Vladimir V. Putin of Russia and to pursue arms accords.

The missile is said to be in violation of a 1987 treaty that bans American and Russian intermediate-range missiles on land. A 1987 treaty bans American and Russian intermediate-range ground launched missiles.

In 2014 it was announced the ground launched cruise missile had been tested.

Former US President Obama had previously tried to convince the Russians to correct the Cold War treaty violation while the treaty was still in its test phase.Instead, the Russians have moved ahead with the system, deploying a fully operational unit.

The US administration now believes Russia has two battalions of the banned weapons.

One is still located at Russia’s missile test site at Kapustin Yar in southern Russia near Volgograd.

The other was shifted in December from that test site to an operational base elsewhere in the country, according to a senior official who did not provide further details and requested anonymity to discuss recent intelligence reports about the missile.

American officials had called the cruise missile the SSC-X-8. But the “X” has been removed from intelligence reports, indicating that American intelligence officials consider the missile to be operational and no longer a system in development.